064
FXUS02 KWBC 170659
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 AM EDT THU OCT 17 2024
VALID 12Z SUN OCT 20 2024 - 12Z THU OCT 24 2024
..OVERVIEW
A CLOSED UPPER LOW ATOP THE FOUR CORNERS INTO SUNDAY COULD LEAD TO
LINGERING SNOW IN THE SOUTH-CENTRAL ROCKIES AND COOL TEMPERATURES,
WHILE MODERATE TO LOCALLY HEAVY RAIN WITH SOME ISOLATED FLOODING
CONCERNS SPREADS INTO THE SOUTH-CENTRAL PLAINS. THE UPPER LOW IS
FORECAST TO TRACK NORTHEAST AND WEAKEN AS THE WEEK PROGRESSES WITH
SOME LIGHTER RAIN. MEANWHILE, PRECIPITATION IS LIKELY IN THE
PACIFIC NORTHWEST EARLY NEXT WEEK WITH THE TAIL END OF AN
ATMOSPHERIC RIVER. BY MIDWEEK AND BEYOND, BROADLY CYCLONIC FLOW
SHOULD SET UP ATOP THE EASTERN TWO-THIRDS OF THE CONUS WHILE UPPER
RIDGING MAY FORM OVER THE WEST IN RESPONSE TO AMPLE ENERGY ACROSS
THE EASTERN PACIFIC. ADDITIONALLY, RIDGING RETROGRADING FROM THE
GULF OF MEXICO THROUGH SOUTHERN TEXAS AND MEXICO WILL SPREAD WARMER
THAN AVERAGE TEMPERATURES TO THE SOUTHERN STATES, WHILE COLD
FRONTS GRADUALLY COOL THE NORTHERN TIER AFTER A WARM EARLY WEEK
PERIOD THERE.
..GUIDANCE/PREDICTABILITY ASSESSMENT
RECENT GUIDANCE IS GENERALLY AGREEABLE WITH THE PATTERN DESCRIBED
ABOVE. THERE ARE SOME MINOR TIMING DIFFERENCES WITH THE EJECTION OF
THE FOUR CORNERS UPPER LOW. THE 00Z CMC SEEMS TO BE A SLOW/FARTHER
WEST OUTLIER WITH THE UPPER LOW EVEN AS THE PERIOD BEGINS SUNDAY
AND MAINTAINS A SLIGHTLY SLOWER FEATURE WHILE RECENT GFS/EC RUNS
ARE MORE AGREEABLE, ALONG WITH THE AI MODELS. BY AROUND TUESDAY THE
UPPER LOW'S TIMING MAY ALSO DEPEND ON THE ARRIVAL OF NORTHERN
STREAM ENERGY PUSHING INTO THE NORTH-CENTRAL U.S., BUT THIS IS
SHOWING BETTER CONSENSUS IN RECENT GUIDANCE.
FARTHER WEST, A SOUTHWEST TO NORTHEAST ORIENTED MEAN TROUGH
STRETCHING FROM SOUTHWESTERN CANADA INTO THE PACIFIC EARLY NEXT
WEEK SHOWS SOME SPREAD WITH ITS EVOLUTION, DEPENDENT IN PART ON
ENERGY DISTRIBUTION WITHIN THE TROUGH AND ADDITIONAL SHORTWAVES
DIVING IN. MODEL GUIDANCE FROM THE LAST FEW CYCLES HAS INDICATED
THAT AN UPPER LOW IS LIKELY TO CLOSE OFF WELL OFFSHORE, PERHAPS
BETWEEN 140W AND 150W LONGITUDE, BUT CREEPING EASTWARD BY
WEDNESDAY-THURSDAY. THIS LEADS TO THE POSSIBILITY FOR MEAN RIDGING
TO SET UP OVER THE WEST COAST TO ITS EAST, AND PROMOTING BROAD
CYCLONIC FLOW TO THE EAST OF THAT RIDGE OVER GENERALLY THE REST OF
THE LOWER 48. THERE ARE UNCERTAINTIES WITH POSSIBLE SHORTWAVE
ENERGY NEAR THE PACIFIC NORTHWEST AROUND WEDNESDAY LINGERING AFTER
THAT UPPER LOW FARTHER WEST PINCHES OFF. MANY DETERMINISTIC AND AI
MODELS SHOW INDICATIONS OF THIS FEATURE, BUT BOTH THE 12Z AND NEWER
00Z ECMWF ARE AMONG THE MOST AGGRESSIVE WITH IT. REGARDLESS OF HOW
AGREEABLE THE MAJORITY CLUSTER MAY BE AT THE MOMENT, THESE FLOW
SEPARATION CASES CAN STILL PRODUCE SIGNIFICANT VARIABILITY DURING
THE EXTENDED RANGE.
THE WPC FORECAST WAS BASED ON A BLEND OF THE DETERMINISTIC 18Z GFS
AND 12Z EC/UKMET/CMC EARLY IN THE PERIOD, WITH DECREASING
PROPORTIONS OF THE DETERMINISTIC MODELS AS THE PERIOD PROGRESSED IN
FAVOR OF THE PRETTY AGREEABLE GEFS AND EC ENSEMBLE MEANS.
..WEATHER/HAZARDS HIGHLIGHTS
THE FOUR CORNERS UPPER LOW WILL PROVIDE FORCING FOR PRECIPITATION
ACROSS SOUTH-CENTRAL PARTS OF THE ROCKIES AND PLAINS INTO SUNDAY.
SNOW COULD LINGER IN THE HIGHER ELEVATIONS OF COLORADO IN
PARTICULAR, THOUGH LIKELY DECREASING IN MAGNITUDE COMPARED TO LATE
THIS WEEK. AHEAD OF THE UPPER LOW, MOISTURE STREAMING IN FROM THE
PACIFIC AND THE GULF WILL PROVIDE AN ENVIRONMENT WITH PWS ABOVE THE
90TH IF NOT 95TH PERCENTILE FOR POTENTIALLY HEAVY RAIN AND STORMS
ACROSS THE SOUTH-CENTRAL PLAINS. A MARGINAL RISK REMAINS IN PLACE
CENTERED OVER THE TEXAS AND OKLAHOMA PANHANDLES INTO THE WESTERN
HALF OF KANSAS, AS HEAVY RAIN MAY CAUSE ISOLATED FLOODING THREATS
ESPECIALLY IF AREAS OF HIGH RAIN RATES TRAIN IN THE SAME LOCATIONS.
SOME SHOWERS WILL SHIFT INTO THE CENTRAL PLAINS/MID-MISSISSIPPI
VALLEY BY MONDAY AND PERHAPS THE OHIO VALLEY TUESDAY, THOUGH WITH
DECREASING INTENSITY.
MEANWHILE, THE PACIFIC NORTHWEST WILL LIKELY SEE PRECIPITATION ON
SUNDAY AS AN ATMOSPHERIC RIVER CONTINUES TO SPREAD MOISTURE INTO
THE REGION, WITH MODEST RAIN AND SNOW AMOUNTS. SOME PRECIPITATION
MAY LINGER INTO MONDAY AND SPREAD FARTHER INLAND BUT WITH LESSER
AMOUNTS. GUSTY WINDS ARE POSSIBLE IN THE ROCKIES AS FRONTS PUSH
THROUGH. THERE IS STILL UNCERTAINTY ABOUT SPECIFICS OF EASTERN
PACIFIC/WEST COAST FLOW ALOFT BY TUESDAY-THURSDAY, BUT MOST
GUIDANCE CURRENTLY SUGGESTS THAT THE NORTHWEST WILL SEE A DRIER
TREND BY THEN WITH UPPER RIDGING SETTING UP.
THE EASTERN U.S. IS FORECAST TO BE GENERALLY DRY THROUGH MUCH OF
THE PERIOD. EASTERLY FLOW ACROSS THE FLORIDA PENINSULA COULD BRING
SOME LIGHT SHOWERS ONSHORE. BY TUESDAY-WEDNESDAY, LIGHT RAIN MAY
PUSH ACROSS THE NORTH-CENTRAL TO NORTHEASTERN U.S. WITH A FRONT.
COOLER THAN AVERAGE TEMPERATURES BY 5-15 DEGREES ARE FORECAST TO
CONTINUE INTO SUNDAY IN THE FOUR CORNERS STATES UNDERNEATH THE
UPPER LOW, BUT GRADUALLY WARM AFTER THAT. MEANWHILE, UPPER RIDGING
AHEAD OF THE UPPER LOW WILL PROMOTE ABOVE NORMAL TEMPERATURES
ACROSS THE NORTHERN PLAINS TO UPPER MIDWEST/GREAT LAKES AND
NORTHEAST. THE MIDWEST CAN EXPECT THE GREATEST ANOMALIES OF 15-25
DEGREES ABOVE AVERAGE ON SUNDAY AND MONDAY AS HIGHS REACH THE 70S.
FORECAST TRANSITION OF THE UPPER PATTERN TOWARD BROADLY CYCLONIC OR
NEARLY ZONAL MEAN FLOW ALONG WITH A SURFACE COLD FRONT WOULD
MODERATE TEMPERATURES OVER THE NORTHERN PLAINS AND VICINITY BY
MIDWEEK AND INTO THE GREAT LAKES REGION BY NEXT THURSDAY, WHILE A
SUBTROPICAL UPPER HIGH ACROSS THE GULF/NORTHERN MEXICO/TEXAS WILL
ALLOW FOR WARMER TEMPERATURES TO EXPAND ACROSS THE THE SOUTHERN
PLAINS INTO SOUTHEAST.
TATE
ADDITIONAL 3-7 DAY HAZARD INFORMATION CAN BE FOUND ON THE WPC MEDIUM
RANGE HAZARDS OUTLOOK CHART AT:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,
QUANTITATIVE PRECIPITATION FORECAST (QPF), EXCESSIVE RAINFALL
OUTLOOK (ERO), WINTER WEATHER OUTLOOK (WWO) PROBABILITIES, HEAT
INDICES, AND KEY MESSAGES CAN BE ACCESSED FROM:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=ERO
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=OVW
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